1. Field of the Invention
The present invention relates to a semiconductor device including a semiconductor element such as a transistor which includes an oxide semiconductor, and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally refers to a device which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Many transistors formed over a glass substrate or the like are manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although a transistor manufactured using amorphous silicon has low field-effect mobility, it can be formed over a larger glass substrate. On the other hand, although a transistor manufactured using polycrystalline silicon has high field-effect mobility, it is not suitable for being formed over a larger glass substrate.
In recent years, other than a transistor formed using silicon, a technique in which transistors are manufactured using an oxide semiconductor and applied to electronic devices or optical devices has attracted attention. A technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide, for example, as an oxide semiconductor and used for a switching element of a pixel of a display device or the like is disclosed in Patent Document 1 and Patent Document 2.
A transistor including an oxide semiconductor film is expected to considerably improve performance of a display device because such a transistor has higher field-effect mobility than a transistor including amorphous silicon.
However, in a semiconductor device in which a transistor needs to have high field-effect mobility, a transistor including polycrystalline silicon or single crystal silicon is used in many cases. In the case where a transistor including an oxide semiconductor is used in a semiconductor device other than a display device, field-effect mobility as high as that of a transistor including polycrystalline silicon or single crystal silicon is needed in some cases.
A transistor including an In—Sn—Zn—O-based oxide and having high field-effect mobility is disclosed (see Non-Patent Document 1).